1. Field of the Invention
The present invention relates to an apparatus for inspecting a photomask (reticle) which is used as a photographic plate for the transferring or printing of a semiconductor circuit pattern, and more particularly the invention relates to a technique of measuring the amount of phase shift in a phase-shifted mask in which a plurality of phase shifters (transparent thin films) for changing the phase of transmitted light are additionally attached to its specified portions.
2. Description of the Prior Art
A mask used as a photographic plate when projecting, exposing and transferring a semiconductor circuit pattern onto the photoresist on the surface of a wafer is generally so constructed that it is made by forming a light shielding pattern of a metal such as chromium on a substrate of glass such as silica. However, such mask of the general construction is disadvantageous in that if a finer circuit pattern is used, it is impossible to ensure a satisfactory contrast for a projected image due to the diffraction of light and the interference phenomenon. Thus, recently a variety of phase-shifted masks have been proposed in which dielectric thin films, called as phase shifters, are deposited at specified locations of a mask pattern to partially change the phase of transmitted light and enhance the contrast of an image. The refractive index and film thickness of the phase shifters are designed in such a manner that a specific phase difference is produced between the light transmitted through the phase-shifter deposited portion and the light transmitted through the non-deposited portion without phase shifter under the exposure light wavelength used in the photolithography.
With such phase-shifted mask, it is important to accurately control the phase thus making it necessary to inspect the amount of phase shift caused by the phase shifters in addition to the presence of damages in the light shielding pattern and it has been the practice in the past to determine the amount of phase shift by use of a thickness gage such as an ellipsometer.
However, the ellipsometer heretofore used for the inspection of a phase-shifted mask performs the film thickness measurement by making use of the multiple reflection from the boundary surface between the mask substrate and phase shifters and the boundary surface between the phase shifters and air and therefore the measurement cannot be performed if the substrate and the phase shifters are equal in refractive index. On the other hand, the exposure light wavelength has been shortened along with the production of finer semiconductor circuit patterns so that in the future it is expected that the far ultraviolet radiation becomes the main current for the radiation source lights. While the number of materials having high transmittances for the ultraviolet radiation is not large and it is conceivable that both the mask substrate and the phase shifters will be made of silica glass, in this case the refractive indices of the two become equal and therefore the measurement of the film thickness of the phase shifters or the amount of phase shift cannot be performed by making use of the ellipsometer.